You can use the keyboard if the buttons on the web page aren't working.
* Copy: Ctrl+C or Ctrl+Insert
* Paste: Ctrl+V or Shift+Insert
* Cut: Ctrl+X or Shift+Delete
See also:
*http://kb.mozillazine.org/Granting_JavaScript_access_to_the_clipboard
*https://addons.mozilla.org/firefox/addon/allowclipboard-helper/